The present invention relates to stereoscopic optical instruments utilizing a liquid crystal, and more particularly, to a liquid crystal director of each of liquid crystal lenses for use in a pair of spectacles and stereoscopic optical apparatus.
The concept of making a lens variable in its focal length utilizing the double refraction of liquid crystal is found in, for example, Japanese Laid-Open Patent Publication Nos. Sho 52/1977-32348, Sho 54/1979-99654 and Sho 58/1983-50339. The application of this concept to spectacles is also indicated in the latter two publications. An example of such conventional lenses employing a liquid crystal (hereinafter referred to as a liquid crystal lens) will be described with reference to a section view of FIG. 15 and a plan view of FIG. 16.
Transparent conductive layers 23 and 24 are provided respectively on a spherical concave lens 21 and a flat glass plate 22. Liquid crystal 26 is enclosed within the space which is formed by joining the conductive layers 23, 24 together through an insulating layers 25 to form a liquid crystal lens 20.
An a.c. voltage from an a.c. power source 27 is applied between the conductive layers 23, 24. When no a.c. voltage is applied, the orientation treatment is applied to the liquid crystal 26 so that its liquid crystal molecules are in the same direction and the liquid crystal molecules in FIGS. 15 and 16 are thus in homogeneous orientation.
When a voltage is applied to the liquid crystal lens 20, molecules of the liquid crystal 26 in the lens 20 rotate so as to orient their longitudinal axis to the direction of the electric field in the case that the dielectic anisotropy of the liquid crystal is positive. Then, when the liquid crystal 20 having a director of liquid crystal molecules which is shown with an arrow n in FIG. 16 is combined with a polarization plate 28 as shown in FIG. 17, so as to allow only extraordinary rays to impinge on the liquid crystal lens 20, the liquid crystal 26 varies from its refractive index to an extraordinary ray n.sub.e to its refractive index to an ordinary ray n.sub.o as liquid crystal molecules within the liquid crystal 20 rotate by the electric field. Now, assuming that a focal length is f, a refractive index is n and radii of curvature of opposite surfaces of a lens are r.sub.1 and r.sub.2 a focal length of a lens is generally shown as follows: ##EQU1## so that it is possible to change the focal length by changing a refractive index n.
With the liquid crystal 20 applied to spectacles, it is possible to continuously change focal length, so that it is greatly useful to those who the focus adjusting function is lowered by presbyopia or is lost by an operation of taking out the eye lens due to cataract.
Such liquid crystal spectacles employing the above conventional liquid crystal lens have disadvantages, however, that it is difficult to adjust the focus or that left and right lenses vary in their changing rates of focal length, thus lacking thoughtful consideration in the human engineering.
This fact will be described hereinafter in detail. A liquid crystal is generally provided with the orientation treatment so as to orient its liquid crystal molecules to a specific direction even before a voltage is applied thereto. The orientation treatment is performed by a rubbing operation with a high polymer film, such as polyimide, polyvinyl alcohol or the like so as to turn to a particular direction or by evaporating SiO.sub.2 MgO, MgF.sub.2 Au or the like from the diagonal direction to the surface of the liquid crystal. As a result, as shown in FIG. 16, the liquid crystal lens 20 becomes an element in which directors of the liquid crystal are oriented to a direction shown with an arrow n.
With a pair of liquid crystal spectacles 30 employing the liquid crystal lens 20, as shown in FIG. 18, in which left and right liquid crystal lenses 31, 32 are incorporated into a spectacle frame so that their respective directors n.sub.L and n.sub.R intersect at right angles, even though voltages from a drive power source 33 to be applied to the lenses 31 and 32 are the same, their focal lengths are different, so that the spectacles 30 are inconvenient to see. Even when the voltages are changed in a similar way, the lens 31, 32 disadvantageously yield their different change rates in focal length.
Such disadvantages are caused, as shown in FIG. 19, by the existence of a pupil distance PD for obtaining stereoscopic vision in human eyes 35, 36 since views through the eyes 35, 36 are different and the difference in projecting rays of light upon the liquid crystal lenses 31, 32 is caused. Further viewing these facts minutely with reference to FIGS. 20A, B and C, the difference in projecting rays of light upon the liquid crystal lens 31, 32 can be regarded as the difference in those upon the liquid crystal molecules. FIGS. 20A and B are top views of the liquid crystal spectacles. FIG. 20C is a front view thereof. Taking the pillar-shaped liquid crystal molecules 37, 38 into consideration by substituting them for the refractive index ellipsoid of the liquid crystal, the liquid crystal 37 within the left eye liquid crystal lens 31 makes an angle .theta..sub.L with a light ray incident upon the liquid crystal lens from an object, as shown in FIG. 20A, and an angle .theta..sub.R between the liquid crystal molecule 38 within the right liquid crystal lens 32 and incident light thereupon is 90.degree.as shown in FIGS. 20B and C. Thus, since .theta..sub.L is not equal to .theta..sub.R the light incident angles to the corresponding refractive index ellipsoids of the liquid crystal lenses 31, 32 are different and hence the lenses 31, 32 yield different infractive indexes.
In other words, even when an object on the extension of the center line of a human body is viewed (front view), the liquid crystal lenses 31, 32 cause the difference in focal length between them to be out of focus.
While the foregoing describes the case where the liquid crystals of the liquid crystal lenses 31, 32 are merely subjected to the initial orientation treatment without applying a voltage thereto, when a voltage is applied they cause the difference in change rate of their refractive indexes, thereby further increasing the difficulty in viewing and in focusing.